Systems for aligning locking screws in intramedullary nails that have been used to secure fractured bones together are known. A description of a number of such systems is set forth in U.S. Pat. No. 5,411,503, which is expressly incorporated herein in its entirety. These systems may be broadly categorized into three classes: x-ray imaging systems, mechanical systems, and electromagnetic systems. X-ray imaging systems use x-ray imaging to provide an image of the limb being treated with the inserted intramedullary nail so the surgeon may view the transverse hole located in the nail. This image facilitates the surgeon's locating the proper position on the external surface of the bone for drilling and aligning the drill bit with the transverse hole. Once the correct drill position and alignment are determined, the x-ray imaging system is removed so the surgeon may then drill a hole through the bone that passes through the hole in the nail. These x-ray imaging systems expose the patient and the surgeon to x-rays and the accumulation of x-rays, especially for the surgeon, may have long term consequences.
The mechanical systems require reference points so the offset distance from the reference point that may be externally determined and viewed by the surgeon correlates to a position that corresponds to the opening of the hole in the intramedullary nail. However, these mechanical systems cannot consistently identify the position and angular orientation of the drill so that the drill bit passes through the transverse hole without engaging the walls of the hole.
Systems that have previously used electromagnetic components for aligning a drill for boring a hole in a bone so the drill bit passes through the transverse hole suffer from a number of limitations. Some systems of this type require that a magnetic dipole be mechanically located within the transverse hole so a magnet dipole on the bone surface aligns with the dipole within the nail. This position may then be marked for drilling, but the angular orientation of the drill must be maintained by the surgeon without further reference to the external dipole that was removed for the drilling operation. Other electromagnetic systems, such as the one disclosed in U.S. Pat. No. 5,584,838, use one or more electromagnetic drive coils and a plurality of electromagnetic flux sensors to guide alignment of a drill bushing with the transverse hole in an intramedullary nail. These systems measure the current or voltage induced in coil sensors associated with a drill bushing by a drive coil that is located within a medullary canal to determine the alignment of the drill bushing axis with the axis of the transverse hole. One limitation of the system disclosed in U.S. Pat. No. 5,584,838 is that the drive coil must be removed from its location within the transverse hole so that the drilling operation may be performed without boring through the drive coil. When the drive coil is removed from the transverse hole the coil sensors no longer generate signals that may be used to align the drill bushing. Consequently, the surgeon must maintain the proper orientation and placement of the drill without any indicia to confirm correct placement of the drill.
One electromagnetic system that overcomes these problems is the system disclosed in U.S. Pat. No. 5,411,503. That system uses a pair of drive coils that are mounted within a probe that is placed within an intramedullary nail after the nail is inserted in the medullary canal of a fractured bone. Two sensor planes mounted in orthogonal relationship to one another are located in fixed relationship with a drill bushing. The drive coils are separated by a distance that corresponds to the offset of the sensor planes from the axis of the drill bushing. The drive coils are oriented so the magnetic fields emitted by the coils are aligned in the same direction but the coils are independently controlled so the field from one of the drive coils does not cut a sensor coil when a magnetic field is being generated by the other drive coil. When the sensor coil planes are aligned with the drive coils so that neither coil induces a current in a sensor coil, the drill bushing is aligned with the transverse hole. The fixed displacement of the sensor planes from the drill bushing is set to place the drill bushing axis in alignment with the transverse hole when no signal is induced in any sensor coil. The forward drive coil is placed very nearly at the transverse hole by engaging a probe stop at the outboard end of the probe with the intramedullary nail when the probe is inserted within the nail.
While the system of U.S. Pat. No. 5,411,503 works well for aligning the axis of a drill bushing with a transverse hole in an intramedullary nail, it suffers from some limitations. For one, aligning a drill bushing axis with a transverse hole so the drill bit enters the hole and passes through the hole without engaging the wall of the hole requires constraining the orientation of the drill bushing so it passes through two points, namely, the entrance and exit openings of the hole. In some applications, a single point, usually the exit point, is all that is required. For example, a surgeon may be able to view one side of a bone and select the position where drilling should begin but be unable to orient the drill bit accurately so it exits the bone at the desired point on the other side of the bone. What is needed is a way of assisting a surgeon in aligning a drill so the drill bit bores through the bone to the exit point as quickly, accurately, and simply as possible.
The system disclosed in U.S. Pat. No. 5,411,503 also requires that the probe and guide are connected to the display via cables. These cables must be sterile since they may come into contact with the blood and bodily fluids from the patient. Additionally, many pieces of equipment in operating rooms have cables and tubes extending from them. Consequently, care must be taken to keep the cables from surgical targeting systems, such as the one disclosed in U.S. Pat. No. 5,411,503, from becoming entangled with the cables and tubes associated with other equipment in the operating room. What is needed is a system that avoids the need for sterilized cables and that reduces the likelihood of cable entanglement.
Another limitation of the system disclosed in U.S. Pat. No. 5,411,503 is a structural weakness introduced into the probe by the placement of the drive coils at the distal end of the probe. The probe is comprised of a main conduit to which the drive coils are mounted at one end and then the main conduit/drive coil assembly is enclosed within a sheath. The drive coils are formed into an integral unit to facilitate probe assembly. The integral unit is formed by orienting the drive coils in the same direction and placing a conduit section of about 1.25 inches in length between the drive coils to stiffen the integral unit. The drive coils and short conduit section are then encapsulated in a polymeric resin with the wires to the drive coils extending in the same direction beyond one end of the drive coil unit. This integral unit is joined by adhesives or the like to the distal end of the conduit with the wires of the integral unit extending down the length of the main conduit. The conduit/integral unit assembly is enclosed within a sheath to form the probe. This probe construction had a structural weakness where the integral unit joined the main conduit. Occasionally, stress on the probe would cause the integral unit to separate from the main conduit at the point where they were joined to one another. What is needed is a way of constructing a drive coil probe that simplifies probe assembly and reduces the likelihood of structural weakness in a probe at a drive coil.